1. Field of the Invention
The present invention relates to a contact switch and an apparatus comprising the contact switch.
2. Description of the Background Art
An electrostatic microrelay which is a form of the conventional contact switch is shown in FIG. 8. In FIG. 8, a perspective view of the electrostatic microrelay is shown in FIG. 8A and its sectional view taken along line b—b is shown in FIG. 8B.
As shown in FIG. 8A, the electrostatic microrelay mainly consists of a fixed substrate 201 comprising a glass substrate or an insulator substrate, and a movable substrate 202 comprising a semiconductor such as silicon (Si).
A fixed electrode 204 coated with an insulating film 203, and two signal lines 205 through which a high frequency signal passes are mainly provided on the one fixed substrate 201. The signal lines 205 are provided so as to be spaced at a predetermined distance and ends of the signal lines provide a couple of fixed contacts 206.
The other movable substrate 202 is fixed through an anchor 207 to be bonded to the fixed substrate 201 so as to be opposed to the fixed substrate 201. In addition, on the movable substrate 202, a movable electrode 208 positioned so as to correspond to the fixed electrode 204 is provided and a movable contact 209 which is electrically insulated from the movable electrode 208 is positioned so as to correspond to the fixed contacts 206.
Then, a first elastic support portion 211 constituted by a notch is formed between the anchor 207 and the movable electrode 208 to elastically support the movable electrode 208, and a second elastic support portion 212 constituted by a notch is formed between the movable electrode 208 and the movable contact 209.
Next, a description is made of operations of the thus constituted electrostatic microrelay according to the prior art.
More specifically, as shown in FIG. 8B, the first elastic support portion 211 and the second elastic support portion 212 are not elastically deformed and a state in which they horizontally extend from the anchors 207 is maintained while a voltage is not applied between the fixed electrode 204 and the movable electrode 208 and electrostatic attraction force is not generated.
Then, when a voltage is applied between the fixed electrode 204 and the movable electrode 208, electrostatic attraction force is generated between them. Then, the movable electrode 208 is drawn to the fixed electrode 204.
Thus, when the electrostatic attraction force acts on the movable electrode 208, the first elastic support portion 211 having elastic force smaller than that of the second elastic support portion 212 is elastically deformed first, and the movable electrode 208 and the movable contact 209 come close to the fixed electrode 204 and the fixed contacts 206, respectively while they keep parallel state thereof. Then, the movable contact 209 comes in contact with the fixed contacts 206 and the two signal lines 205 are electrically connected.
Then, the movable electrode 208 is drawn by the electrostatic attraction force and sticks to the fixed electrode 204. Thus, the second elastic support portion 212 is elastically deformed. Then, the movable contact 209 is pushed to the fixed contacts 206 by spring elasticity caused by the deformation of the second elastic support portion 212.
Thus, according to the electrostatic microrelay, when it is closed, the movable contact 209 and the fixed contacts 206 are closed by two-stage elastic deformation in which the first elastic support portion 211 is elastically deformed first and then, the second elastic support portion 212 is elastically deformed.
When the voltage is cut off, the electrostatic attraction force disappears. Thus, the movable substrate 202 is separated from the fixed substrate 201 by force of restitution of the first elastic support portion 211 and the second elastic support portion 212 and returned to the original position. Accordingly, the movable contact 209 is vertically lifted by this restitution force to be separated from the fixed contacts 206 and electrical connection between the two signal lines 205 is cut off.
In addition, a cap 210 formed of glass is bonded on the upper surface of the fixed substrate 201 through a bonding layer (not shown) in order to protect the movable substrate 202 from an outside foreign substance such as dust.
However, the following problems arises in the contact switch such as the electrostatic microrelay according to the prior art.
Spring design in the contact switch is designated by that F=kx (k: elastic coefficient, x: stroke amount). Therefore, in the case of the above microrelay, the necessary stroke amount is defined by a gap amount between the movable contact 209 and the fixed contacts 206.
However, the gap amount between the contacts is influenced by film thickness variation in the film of the fixed contacts 206 in the device manufacturing process of the contact switch, thickness variation of an insulator for insulating the movable contact 209 from the movable electrode 208 and a conductor for constituting the movable contact 209, and processing precision at the time of processing the contacts.
In this respect, according to the knowledge based on various kinds of experiments performed by the inventor of the present invention, the precision variation in the above described variations is largest at the portion of the fixed contacts 206 (in the circle designated by a dotted line in FIG. 8) formed by the thickest film.
Meanwhile, since the signal lines 205 transmit the high frequency signal with low loss as much as possible, the thickness of the wiring has to be a skin depth or more in consideration of a skin effect.
When variation of the gap amount between the contacts is generated, contact reliability between the movable contact 209 and the fixed contacts 206 in the electrostatic microrelay is influenced.
More specifically, when the gap amount between the contacts is greater than a design value, a distance between the movable electrode 208 and the fixed electrode 204 (distance between electrode gaps) when the movable contact 209 and the fixed contacts 206 are closed and come in contact with each other is smaller than the design value.
Thus, a displacement amount of the movable electrode 208 from the state in which the contacts are closed until the state in which the fixed electrode 204 and the movable electrode 208 come in contact with each other by electrostatic attraction force becomes small, and a displacement amount of the second elastic support portion 212 which starts spring deformation from the state in which the contacts are closed becomes small also. Here, since the deformation of the second elastic support potion 212 is generated while from the state in which the contacts are closed until the electrodes come in contact, force acting on the movable contact 209 from the second elastic support portion 212 is designated by the above described spring design based on the displacement amount set on the basis of the state in which the contacts are closed.
Thus, based on this spring design, the elastic force acting on the second movable contact 209 is decreased because the displacement amount of the movable electrode 208 is decreased. Consequently, the movable contact 209 cannot be sufficiently pushed toward the fixed contacts 206 so that contact reliability cannot be secured.
Meanwhile, when the gap amount between contacts is smaller than the design value, the distance gap between the movable electrode 208 and the fixed electrode 204 when the movable contact 209 and the fixed contacts 206 are closed to be in contact with each other is more than the design value.
Thus, electrostatic attraction force acting on the movable electrode 208 toward the fixed electrode 204 is reduced. When the electrostatic attraction force becomes smaller than the sum of the elastic force of the first elastic support portion 211 and the second elastic support portion 212, a phenomenon in which the fixed electrode 204 and the movable electrode 208 do not contact with each other could occur.
When the fixed electrode 204 does not come in contact with the movable electrode 208, since the elastic displacement amount of the second elastic support portion 212 is reduced, the movable contact 209 cannot be sufficiently pushed toward the fixed contacts 206 by the second elastic support portion 212, based on the above described spring design. In this case also, there arises a problem that the contact reliability between the contacts cannot be obtained.